The present invention relates to communication networks. In particular the present invention relates to a method and system for reducing the peak to average power ratio of wireless signals.
It is predicted that the 21st century will witness the widespread deployment of wireless networks that will revolutionize the concept of communication and information processing for business, professional and private applications. However, bandwidth scarcity and a hostile radio environment are among the two most significant technical hurdles for developing the next generation of wireless information systems. The latter issue is especially problematic in developing broadband wireless networks.
In particular, multipath delay spread resulting in intersymbol interference imposes an absolute limit on the bandwidth of a wireless channel. Orthogonal frequency division multiplexing (OFDM) is a very attractive technique for achieving high-bit-rate transmission in a radio environment. By dividing the total bandwidth into many narrow subchannels, each carrying a lower bit rate, which are transmitted in parallel, the effects of multipath delay spread can be minimized. Thus, the problem of intersymbol interference can be solved by increasing the symbol duration in the same ratio as the number of subchannels. This approach has been proposed or adopted for many wireless applications including digital audio broadcasting, digital terrestrial television broadcasting, wireless LANs and high-speed cellular data. Techniques for implementing OFDM are well known.
However, a significant disadvantage of employing OFDM for wireless applications is the potentially large peak-to-average power ratio (PAP) characteristic of a multicarrier signal with a large number of subchannels. In particular, a baseband OFDM signal with N subchannels has a PAP of N2/N=N, for N=256, PAP=24 dB. When passed through a nonlinear device, such as a transmit power amplifier, the signal may suffer significant spectral spreading and in-band distortion. With the increased interest in OFDM for wireless applications, reducing the PAP is a necessity for implementing OFDM.
For wireless applications, efficient power amplification is required to provide adequate area coverage and to minimize battery consumption. The conventional solution to the PAP problem in OFDM systems is to use a linear amplifier that is operated with large backoff from its peak power limit. However, this approach results in a significant power penalty.
Several alternative solutions have been proposed to reduce the PAP. For example, one simple solution is to deliberately clip the OFDM signal before amplification, which provides a good PAP but at the expense of performance degradation. See R. O""Neill and L. N. Lopes, xe2x80x9cEnvelope Variations and Spectral Splatter in Clipped Multicarrier Signals,xe2x80x9d Proc. of PIMRC""99, pp. 71-75.
Another known conventional solution is nonlinear block coding, where the desired data sequence is embedded in a larger sequence and only a subset of all possible sequences are used, specifically those with low peak powers. See A. E. Jones, T. A. Wilkinson, and S. K. Barton, xe2x80x9cBlock Coding Scheme for Reduction of Peak to Mean Envelope Power Ratio of Multicarrier Transmission Scheme,xe2x80x9d Electron. Letts., Vol. 30, No. 25, December 1994, pp. 2098-2099. Using this nonlinear block coding approach, a 3-dB PAP can be achieved with only a small bandwidth penalty. However, the drawback of nonlinear block coding is that it requires large look-up tables at both the transmitter and receiver, limiting its usefulness to applications with only a small number of subchannels. There has been progress in developing coding schemes that reduce the PAP, can be implemented in systematic form, and have some error correcting capabilities. See A. E. Jones and T. A. Wilkinson, xe2x80x9cCombined Coding for Error Control and Increased Robustness to System Nonlinearities in OFDM,xe2x80x9d Proc. of VTC""96, pp. 904-908. Nevertheless, these coding methods are difficult to extend to systems with more than a few subchannels and the coding gains are small for reasonable levels of redundancy.
Two promising techniques for improving the statistics of the PAP of an OFDM signal have been proposed. These techniques have been termed the selective mapping (SLM) approach and the partial transmit sequence (PTS) approach.
In selective mapping, M statistically independent sequences are generated from the same information and that sequence with the lowest PAP is chosen for transmission. To recover the data, the receiver must xe2x80x9cknowxe2x80x9d which sequence has been used to xe2x80x9cmultiplyxe2x80x9d the data; this can be transmitted as side information.
In the PTS approach, each input data block consisting of a set of subcarrier coefficients is partitioned into disjoint subblocks, which are then combined to minimize the PAP. Specifically, each subcarrier coefficient is multiplied by a weighting coefficient, or phase factor. The phase factors are chosen to minimize the PAP of the transmitted signal.
Although both the selective mapping approach and the partial transmit sequence approach are useful for improving the statistics of the PAP of an OFDM signal, both introduce additional implementation complexity. In particular the SLM approach requires the use of M full-length (i.e., N-point) IFFTs (Inverse Fast Fourier Transforms) at the transmitter. The PTS approach requires a similar number of IFFT""s and in addition introduces additional complexity due to the requirement of optimizing the assignment of phase factors to each partial transmit sequence. This computational complexity imposes limitations on battery life, particularly in the terminal unit. Thus, there is a need for a method to reduce the PAP of a signal that can be performed with low computational complexity.
Code Division Multiple Access (CDMA) is another very attractive technique for overcoming the bit rate limitations of the multipath channel. In addition, one of the approaches for achieving higher (as well as variable) bit rates consists of individual terminals transmitting multiple CDMA codes (multi-code CDMA). In both basic CDMA and multi-code CDMA, a similar PAP problem exists and a method for reducing the PAP of such a signal is desirable.
The present invention provides a method and system for reducing the PAP of a signal with low complexity compared to existing techniques. According to one embodiment, the present invention is applied to reduce the PAP of an OFDM signal. In an alternative embodiment, the present invention is applied to reduce the PAP of a CDMA signal. Rather than seeking the optimum solution, which involves significant computational complexity, the present invention provides for a number of sub-optimal techniques for reducing the PAP of an OFDM signal but with much lower computational complexity. In particular, according to one embodiment utilizing the PTS approach, an iterative technique is used to assign phase factors to each of a set of partial transmit sequences from a set of possible phase factors. Experimental results using the iterative technique showed only a slight degradation (1 dB) from the optimal approach using the same number of subblocks and subcarriers. In an alternative embodiment, which avoids feedback required by the iterative approach, a sequence of phase factors are generated randomly and assigned to each of a set of partial transmit sequences. This procedure is repeated for a pre-determined number of trials and the random sequence generating the lowest PAP is selected. In a third embodiment, a set of phase factors is generated using a structured sequence such as a Walsh sequence.